The present invention relates generally to transfer cases for use in four-wheel drive vehicles. More particularly, the present invention relates to a full-time transfer case having a two-speed gear reduction unit and an interaxle differential integrated into a planetary gear assembly and a synchronized range shift mechanism for permitting on-the-fly shifting between high-range and low-range drive modes.
As is now conventional, many light-duty and sport-utility vehicles are equipped with a transfer case for transmitting drive torque to all four of the wheels, thereby establishing a four-wheel drive mode of operation. To accommodate differing road surfaces and conditions, many transfer cases are equipped with a gear reduction unit which can be selectively shifted to permit the vehicle operator to choose between a four-wheel high-range (i.e., direct ratio) drive mode and a four-wheel low-range (i.e., reduced ratio) drive mode. In many instances, the four-wheel drive vehicle must be stopped before the transfer case can be shifted between its four-wheel high-range and low-range drive modes. Unfortunately, the need to stop the vehicle prior to shifting between the available four-wheel high-range and low-range drive modes is inconvenient, particularly upon encountering road conditions or surface terrains where continuation of the vehicle's rolling momentum would assist in overcoming the conditions encountered. To alleviate a portion of this inconvenience, some gear reduction units have been designed which permit the vehicle operator to shift without stopping the vehicle (i.e., "on-the-fly") from the four-wheel low-range drive mode into the four-wheel high-range drive mode. For example, U.S. Pat. No. 5,054,335 discloses a transfer case equipped with a synchronized range shift arrangement for "on-the-fly" shifting of a layshaft-type gear reduction unit. Alternatively, commonly-owned U.S. Pat. No. 5,346,442 discloses a transfer case having a synchronized range shift arrangement for "on-the-fly" shifting of a planetary-type gear reduction unit. Finally, U.S. Pat. No. 4,569,252 discloses a planetary-type gear reduction unit which permits synchronized shifting into and out of the high-range drive mode and the low-range drive mode.
In addition to the gear reduction unit, many transfer cases are also equipped with a mode shift mechanism which permits the vehicle operator to selectively shift between a two-wheel drive mode wherein only the rear driveline is driven and a "part-time" four-wheel drive mode wherein the front driveline is rigidly coupled for rotation with the rear driveline. Reference may be made to commonly-owned U.S. Pat. No. 4,770,280 for disclosure of an exemplary part-time transfer case equipped with a gear reduction unit and a synchronized mode shift mechanism. In view of increased consumer popularity in four-wheel drive vehicles for everyday use, the mode shift mechanism in some two-speed transfer cases is replaced with an interaxle differential which is operable for continuously transmitting drive torque to both the front and rear drivelines while permitting speed differentiation therebetween. Reference may be made to commonly-owned U.S. Pat. No. 4,677,873 for disclosure of an exemplary full-time two-speed transfer case equipped with a gear reduction unit and a dual-planetary interaxle differential. In order to prevent traction loss due to excessive interaxle slip, some full-time transfer cases are also equipped with a limited slip device which is operable for selectively or automatically biasing the torque distribution ratio across the interaxle differential while preventing or regulating speed differentiation in response to the slip condition. In some systems, a lock-out clutch is manually-actuated to lock the interaxle differential. However, other full-time transfer cases are equipped with a hydraulic coupling or an electronically-controlled bias clutch for automatically regulating the amount of allowable speed differentiation across the interaxle differential.
In an effort to minimize the overall size of full-time transfer cases, it has been proposed to incorporate the gear reduction unit and the interaxle differential into a common planetary gear assembly. One such arrangement is described in U.S. Pat. No. 4,644,822 which discloses a transfer case having "back-to-back" first and second planetary gearsets with common components that can be selectively shifted to establish different four-wheel drive modes. Likewise, each of commonly-owned U.S. Pat. Nos. 5,284,068 and 5,411,447 discloses a full-time dual-speed transfer case equipped with an integrated planetary gear assembly that is shifted as a unit to establish the different available drive modes. Finally, U.S. Pat. Nos. 4,677,875, 4,344,335, 4,215,593 and 4,805,484 each disclose a transfer case having a common planetary gear assembly which is operable to establish various drive modes through manipulation of one or more suitable shift mechanisms. While such prior art arrangements provide a compact construction, there is a continuing need to develop low cost, simplified alternatives which meet modern requirements for low noise and weight.